In modern paper machines the paper is manufactured in a continuously traveling web. The paper web width might be as much 10 m and the web is traveling at high speeds such as 1000 m/min and more and the length of the paper machine itself might be over 100 m.
In a paper machine of this type a paper suspension is initially supplied to the inlet box of the machine and on its passage through the machine the paper suspension is dehydrated, pressed, dried and so on to a continuous paper web which is finally winded up on rolls in the reel-up gear.
Existing paper machines are operated continuously and they are controlled by computers. From a number of measuring points current operating parameters for the paper such as pressure, temperature, surface weight, thickness, humidity and so on, are supplied to the computer. Based on these parameters the computer is controlling the paper manufacturing process by adjusting valve condition, machine speed and the like so that a high quality of the produced paper web can be maintained.
As a modern paper machine might produce as much as 250 000 ton of paper a year and the cost for such a machine is about 2 billion Swedish crowns it should be understood that is very important to monitor the quality of the paper produced by the machine during the entire manufacturing process. An interruption or other breakdown in production might give rise to very serious economical consequencies.
One critical phase in the paper making process is the change-over procedure from a webbed-up tambour to a new, empty tambour in the reel-up gear during the continuous winding. Even if the pressure of the rolls have been continually monitored also in previous available systems usually the first few meters of the web on the new roll have been damaged.
Probably the most important parameter in paper winding is the nip force which is the force between the tambour and the pope. In existing systems this force has been measured indirectly by means of a number of pressure transducers used for controlling the pressure in the hydraulic system of the machine. Measuring this pressure, however, does not give an accurate value of the real nip force. This depends on the fact that during the change-over procedure when the rolls are changed a number of adjustments are made to switch the paper web to the new tambour and during these adjustment operations comparatively big alterations in the nip force level might occur and which cannot always be compensated for by the pressure control system.
The change-over procedure for a paper roll can be divided into a number of phases or positions, such as
(1) a reeling phase for winding the paper web to a completed tambour, PA1 (2) a loading position in which a tambour is completed and the new tambour core is brought into its wound roll winding position, PA1 (3) a change-over position in which the completed tambour is removed and the paper web is switched over to the waiting tambour core, and PA1 (4) a delivery position in which the tambour is switched over from the primary arms to the secondary arms.
The reels are positioned by means of so-called primary- and secondary arms. In present pressure control systems it has been difficult to accurately control the position of the arms during the different phases. This depends on the fact that the pressure transducers have been located on a distance from the arms in the control system.